In general, TeX macros can perform any arbitrary computation
permitted by TeX, a Turing-complete language. Hence, it is
impossible to directly translate the macro expansion into an
audio rendering. The TeX primitives are visual layout
operators, and translating a TeX macro directly into an audio
rendering rule would imply a one-to-one mapping between the
visual and audio rendering. As explained in s:introduction,
visual renderings are attuned to a two-dimensional display, and
audio renderings need to be attuned to an auditory display.
Further, expanding a TeX macro loses structural information;
when all macros in a document have been expanded, only the
visual layout remains.